In recent years, storage devices configured for fast data processes, for example, SD memory card and memory stick, are becoming increasingly popular among consumers. These nonvolatile storage devices, in which a plurality of flash memories are provided as nonvolatile storage cells, are used in hosts of personal computers, PDA (Personal Digital Assistant), mobile telephones, digital cameras, audio players, and automotive navigation systems.
The ongoing price reductions of the flash memories are accelerating the widespread of SSD (Solid State Drive) mounted with a large number of flash memories as an alternative to hard disc drives in personal computers and data centers.
The SSD is a drive device in which flash memories are used as a storage medium. The SSD having the same connection interface as that of the hard disc drive (HDD) (for example, ΔTA) is used as an alternative to hard discs. The SSD may be called flash drive or flash memory drive. The SSD having no mechanically driven parts is very resistant to vibrations during reading and writing processes. The SSD enables the data reading and writing to be processed at high speeds because of no need for time for a reader (head) to move on a rotating disc (seek time) or time for waiting any wanted data on the rotating disc to arrive at a head position (search time).
A flash memory storage device system has a plurality of flash memories and a flash memory controller which controls the flash memories. The flash memory controller is connected to the flash memories by a control bus and a data bus. The flash memory controller selects arbitrary one of the flash memories to read data from and write/erase data in the selected flash memory in response to commands received from the host of the storage device.
For example, a required power supply voltage for data reading is 5V at a maximum, a required power supply voltage for data writing/erasing is 20V at a maximum, and a required power supply voltage for verifying is 10V at a maximum. Because of these differently required voltages, a power supply circuit for feeding the flash memories with power is provided with power supply circuits for process execution which respectively generate a plurality of different voltage levels needed to enable a plurality of different processes to be executed. The power supply circuits for process execution are more specifically a power supply circuit for reading, a power supply circuit for writing/erasing, and a power supply circuit for verifying. These power supply circuits for process execution include booster circuits. The processes are executed in the flash memories by each page or block of a memory cell array. However, it hardly succeeds to complete the processes respectively executed in the target cells in a single attempt by simply injecting and withdrawing electric charges to and from floating gates. Therefore, it is verified in each data writing or erasing whether the data is properly written or erased. The verifying process is repeated until the data is properly written or erased with the power supply voltages being adjusted. A controller provided in the flash memory controls a sequence of processes.
In a storage device used in a host or a slave device, it is necessary to access a plurality of flash memories at once to improve a processing speed. These days, peak currents of external equipments are becoming higher because of large increases of an operating current in each flash memory as a result of the ongoing miniaturization of flash memories and needs for higher operation speeds of the external equipments to keep abreast of hosts increasingly speeding up. A markedly high current capacity is now demanded to feed the flash memories with power, which is largely different to current capacities averagely required. Further, there are other problems to be solved; system crash and degraded data reliability in the flash memories due to overload due to current supply beyond a designed capacity.